Improvements in or relating to power tools

Abstract: A method of controlling a drill/driver, the drill/driver including a rotatable shaft arranged to mount a screwdriver and bit and a motor for rotatably driving the shaft, the motor being driven only during determined conduction periods, the method arranged to provide, within a given time period, a value which is a function of the conduction period necessary to maintain a given motor speed, and wherein the motor is stopped if this value has exceeded a predetermined amount before the end of the given time period.

Description: This invention relates to power tools, for example hand-held power tools and bench-mounted power tools. The invention has particular reference to hand-held and bench-mounted power screwdrivers and power drills with a screwdriving facility, and which are referred to as "drill/drivers" herein and, in particular, to a method of controlling a drill/driller according to the preamble of claim 1 (see FR-A-2 087 583). The efficient and economic use of power tools depends upon the correct selection of the conditions under which the tool is to be operated. For example, in a power drill, the speed of rotation of the drill bit depends upon the drill size and the material of the workpiece upon which the drill is to be used. In the case of screwdriving, the rotational speed of the screwdriver is determined in accordance with the screw size. It is also important to be able to stop the tool when the screw has been screwed home fully to avoid overloading the tool driving motor or damage the workpiece. It has been proposed to employ electronic means to determine, in accordance with information fed to the means by a user, the rotational speed of the motor driving the tool holder of the power tool, such means also maintaining the set speed at a constant value and monitoring the conditions under which the power tool is operating. In particular, the means may monitor the torque exerted by the tool and operate to de-energise the power tool motor in the event that the torque exceeds a safe value, but the prior proposals have not been totally effective. FR-A-2087583 describes a device for varying and regulating the speed of a motor supplied with alternating current. The device includes a thyristor which closes the supply circuit to the motor for certain periods only. Speed regulation is effected by measuring the average current in the motor circuit and applying a voltage in negative feedback to the thyristor circuit to maintain the constant speed. If the current reaches a high value involving risk of damage part of the voltage is applied to block ignition of the thyristor. Accordingly, it is an object of the present invention to provide a method of controlling a drill/driller with a more effective torque sensing and control arrangement. The present invention employs the results of observations of the current which passes through the motor of the power tool during operation, and specifically of the current passing through the motor of a drill/driver as herein defined as it operates to screw a screw into a workpiece. In this respect reference is made to Figure 2 of the accompanying drawings, which is a graph showing changes in motor current (vertical axis) against time (longitudinal axis) that take place during a screw driving operation. The contour of the graph is substantially the same for all permitted screw sizes but the actual current values will depend on the screw size. During period T1, there is a gradual increase in motor current as the screw penetrates the surface of and starts to enter a workpiece, there being a gradual increase in the resistance to rotation offered to the screw. There then follows a time period T2 during which the current flow is substantially constant as the screw thread enters and is driven further into the workpiece. At the end of time period T2, the screw is almost fully home and the screw head engages the surface of the workpiece. As the pressure between the screw head and the surface increases, there is a rapid increase in motor current over the time period T3 at the end of which the screw is fully home and it is then essential to de-energise the tool motor. The present invention provides a method of controlling a drill/driver, the drill/driver including a rotatable shaft arranged to mount a screwdriver and bit and a motor for rotatably driving the shaft, the motor being driven only during predetermined conduction periods, characterised in that the method comprises: (i) providing means that (a) monitors the conduction period required to maintain a constant motor speed within a given time period and (b) calculates first and second values, each of which is calculated using the said monitored conduction period, the first value being a function of the rate of change of the said conduction period necessary to maintain the given motor speed; and (ii) stopping the motor if the said first value has exceeded the said second value before the end of the given time period. By way of example only, an embodiment of hand-held power drill/driver according to the present invention will now be described in greater detail with reference to the accompanying drawings of which: Figure 1 is a block schematic of the control means of the drill/driver, Figures 2 and 3a/b are explanatory diagrams, Figure 4 illustrates schematically a shift register according to the present invention; Figure 5 shows a flow chart of the present invention; Figure 6 illustrates schematically typical data held within the register of Figure 4 when there is no load on the drill/driver; Figures 7 and 8 illustrate schematically typical data held within the register of Figure 4 when the drill/driver is screwing a screw into a workpiece. Figure 1 is a block schematic diagram of the tool motor control circuit and comprises a microprocessor 30, for example type 50927 made by the Mitsubishi Company of Japan. Block 33 represents a reset circuit whose function is described below. Block 34 is the LCD 10 whose segments are energised directly from microprocessor 30 as shown. The control circuit is energised from a mains supply at terminals 35, 36 via a power supply unit shown as block 37 and which drops the mains voltage to levels required by the control circuit and at the same time provides the conventional voltage stabilisation. Connected across the terminals 35, 36 is the tool driving motor armature 38 and its series connected field windings 39. The trigger-operated motor control switch is shown at 40. The supply of power to the motor is regulated by a triac 41 that is series connected with the motor and whose firing angle is controlled by the microprocessor 30 via a control circuit shown in Figure 1 as block 42. In this exemplary embodiment, the control circuit 42 comprises a programmable timer which is supplied with a 4-bit time delay input from the micro-processor 30. The time delay corresponds to the phase angle delay at which the triac 41 is to be triggered into conduction. Regulation of the rotational speed of the motor is effected by the microprocessor in conjunction with a motor shaft speed measuring circuit represented by block 43. Circuit 43 receives an input from a speed sensing device (not shown) associated with the motor shaft and which may be an optical sensor comprising a slotted disc mounted on the shaft and a light source arranged to direct light through the slotted disc onto a light responsive device. Alternatively, the sensor may be a form of magnetic sensor, for example a Hall effect type sensor, or a coil. The control circuit also provides soft start on all operations, anti-kick back protection in all modes, rotation in the reverse direction being permitted only in the screwdriving mode and following operation of the anti-kick back protection. Also included in the control circuit is thermal protection of the motor by means of a thermistor or thermostat shown as block 44 that provides a signal indicating motor temperature. When the drill/driver is operating in the drilling mode, the control circuit permits the user to select the optimum speed for the drill size and work piece material and maintains that speed despite permitted variations in the load on the motor. When the drill/driver, is operating in the screwdriving mode, the control circuit allows selection of small or large screws and automatic torque control. In the hole cutting mode, optimum speed selection and control thereof is provided. In both polishing and wirebrushing modes the control circuit maintains the motor speed at a constant value despite permitted variations in the load on the motor. Reference 45 represents information contacts on the reversing switch. As has been explained above, when a screw driving operation is being carried out, it is important to be able to stop the tool motor when the screw has been driven fully home. The drill/driver incorporates torque control to achieve that facility. Figures 3(a) and 3(b) are waveforms showing the triac conduction patterns when (a) the driver is driving the shank of the screw into the workpiece and (b) the head of the screw comes into contact with the workpiece. In these waveforms, the lines F1...F6 represent a set of firing angles, the firing angles being a measure of the time the triac is on (i.e. the triac conduction time) in order to turn the chuck of the driver at a certain predetermined speed, e.g. 200 rpm. As load is applied to the chuck the conduction time is increased, in order to maintain the required speed. Thus, while the shank is being driven into the workpiece, Figure 3(a), the conduction time (or firing angle) represented by the hatched portion is small. When the screw head reaches the workpiece, however, a substantial increase in conduction time would be required to maintain the speed, as shown by the hatched portion in Figure 3(b). The microprocessor 30 is programmed to recognise the sudden change from the Figure 3(a) situation to that of Figure 3(b) and stop the motor when this occurs. Thus, as mentioned, the lines F1....F6 on Figures 3(a) and (b) represent a set of firing angles of the triac, and by monitoring the rate of increase in the firing angle it is possible to detect accurately the instant at which it is desirable to de-energise the tool motor at the end of a screwdriving operation. Thus, each of the firing angles F1...F6 is defined by a value, in this example a 4-bit code, for example angle F1 is defined by the code 0000, F2 by the code 0001, F3 by the code 0011 and so on. The definition of the firing angle is effected by the programming of the microprocessor. The microprocessor is also programmed to monitor the rate of change of firing angle. To bring the drill/driver into use, it is first necessary to connect it to a source of mains power. That initiates a resetting of the control circuit by a signal from the reset circuit 33. Resetting involves the clearing of all memories in the control circuit and the setting up of all input and output ports and interrupts. Figure 4 illustrates schematically a sixteen-stage serial memory shift register 114. The register 114 is part of the microprocessor 30 and operates under the control thereof. In addition to supplying the programmable timer within the control circuit 42 with a time delay input, the microprocessor 30 also supplies this time delay to the register 114 in the following manner. Each half-cycle during which the triac 41 may be fired, as illustrated in Figures 3a and 3b, is of 10 ms duration. This 10 ms half-cycle corresponds to 500 counts made by the microprocessor 30. So if, for example, there was a delay of 2 ms before the triac 41 was fired during any given half-cycle, this would correspond to a count of 100. It will be noted that it is the delay before firing rather than the duration of the firing itself of the triac 41 which is being measured. Referring once again to Figure 4, it will be seen that into each of the sixteen stages of the register 114 is inputted from the microprocessor 30 the count which corresponds to the delay before firing the triac 41 for that half-cycle. Thus the most recent (i.e. the current count) count occupies stage number 1 and the oldest count occupies stage number 16. It will be apparent at the time span over which the shift register 114 occupies memory is 160 ms (16 stages each of 10 ms duration) and that the shift register is of the last-in-first-out (LIFO) type. The counts within each of the stages of register 114 is, as in the previous example, the time delay before firing of the triac 41 necessary to maintain the shaft of the drill/driver rotating at a constant speed, here 200 r.p.m. Once the drill/driver has been energised for a sufficient length of time for all stages of the register 114 to hold a respective count, then the microprocessor performs the following calculations necessary to determine whether to continue the energisation of the drill/driver or to stop it. For each count update of the shift register 114, the microprocessor 30 divides the sixteen stages of the register 114 into two halves. One half comprises the eight oldest counts, 8 OLD , and the other half comprises the eight newest counts, 8 NEW . The microprocessor 30 then calculates the average of all the 8 OLD counts, 8 OLD , and also the average of all the 8 NEW counts, 8 NEW . This results in two derived values 8 OLD and 8 NEW . Next the following calculation is performed 8 OLD - 8 NEW → RESULT The value RESULT is a single number and is used for comparison with a further derived value, as will be described below. It will, however, be apparent that because each of the counts in the register is 114 is actually a measure of the delay before firing the triac 41 then, as the shank of a screw is driven further into a workpiece and thus the resistance offered by the screw to the drill/driver increases, then the delay before firing must be concomitantly reduced in order to maintain a steady 200 rpm, as described with reference to the previous embodiment. Hence, as the screw takes further and further into the workpiece, then the values 8 NEW and 8 OLD become smaller. However, because the value RESULT is calculated as the difference between values 8 OLD and 8 NEW over a given period of time, the RESULT will not itself change unless the rate of change between 8 OLD and 8 NEW changes. This achieves a measure of the gradient as shown previously with reference to Figure 2. As has been stated above, it is necessary for the derived value RESULT to be compared with another derived value in order for a 'judgement to be made as to whether or not to de-energise the drill/driver. This derived value, SENSITIVITY is calculated as follows: where K is a number and is a constant for a particular drill/driver and 8 NEW is the average value of the counts shown in Figure 4. The microprocessor 30 de-energizes the drill only if the following condition holds: RESULT > SENSITIVITY This decision making process is illustrated by the flow chart shown in Figure 5. If the condition that RESULT > SENSITIVITY does not hold, then the whole process described above is repeated, but with the counts in the register 114 updated by one stage, i.e. by another half-cycle. This means that all previous values 1-15 move one stage further along the register 114, with value 16 being discarded and a new value at stage 1 being input by the microprocessor 30. Referring now once again to the derived value SENSITIVITY, it will be appreciated that in some circumstances it may be desirable to give different drill/drivers different levels of sensitivity, that is to place constraints on the rate of increase of current, or parameter related at least in part thereto, which can be tolerated before de-energising takes place. For example, as has been described hereinbefore, the actual current values through the motor will depend upon the screw size. It will also depend on the hardness of the workpiece into which the screw is being driven. Thus, for a small screw being driven into a soft wood, a much smaller current and shallower gradient of curve (see Figure 2) will be seen than for a large screw being driven into a hard wood. However, as has been described above, if the drill/driver, on driving the shank of the large screw into hard wood, detects changes in resistance offered by the screw (due to, say, inhomogeneties in the wood) sufficient to hold good the requirement that RESULT > SENSITIVITY, then the drill/driver will be de-energised. This change in resistance offered by the large screw may, in absolute terms, be the same as would be offered by the head of a small screw coming into contact with the surface of soft wood. So, if the drill/driver were actually being used to drive a small screw into soft wood, a satisfactory result would have been achieved. However, if the drill/driver had been driving a large screw into hard wood, an undesired de-energising would have resulted. Thus, by offering the capability of changing the value of K for different drill/drivers, the above problem could be alleviated. For example, K could be set such that the problem outlined above could be altered so that only large screws are to be used with the particular drill/driver in question. This is because the resistance offered by the head of the small screw engaging a worksurface would not be sufficient to cause RESULT > SENSITIVITY to hold due to the change in SENSITIVITY caused by altering the value of K. Examples of values used in the above calculations are shown below and with reference to Figures 6-8. It will be noted that in each of these Figures the data held within the shift register 114 is actually held both in hexadecimal and in decimal. The value of K has been chosen here to be 400. Figure 6 shows both the hexadecimal count and the corresponding decimal count for each of the sixteen stages in the shift register 114 for a drill/driver being run under a no-load condition, i.e. the drill/driver is energised but being run in free space without any resistive load engaging the chuck 4. The decimal values are then split into the 8 OLD and 8 NEW values as described hereinbefore. The calculations necessary to decide whether or not to de-energise the drill/driver are then undertaken. As can be seen by referring to the decimal column of Figure 6, 8 OLD = 372 and 8 NEW = 371 So 8 OLD - 8 NEW = 372 - 371 = 1 ∴ RESULT = 1 And the value with which RESULT is to be compared, SENSITIVITY, is derived as follows: Thus RESULT < SENSITIVITY and so the drill/driver's not de-energised and so all the values in the stages of the register 114 are scrolled on by one place losing the oldest value and providing a new current, or as in Figures 6-8, ACTUAL value. Figure 7 illustrates further examples of hexadecimal and decimal counts. Figure 8 shows the case of a drill/driver performing a screwing operation along the region T2 in Figure 2. Here it can be seen that 8 OLD = 348 and 8 NEW = 338 Thus 8 OLD - 8 NEW = RESULT = 10 and Thus RESULT < SENSITIVITY and so, again, the driver is not de-energised. In another case (not illustrated in the Figures) during the transition from region T2 to region T3 of Figure 2, the counts are as follows: 8 OLD = 357 and 8 NEW = 317 thus RESULT = 8 OLD - 8 NEW = 357 - 317 = 40 and SENSITIVITY = (400 - 317) / 4 = 20 Thus RESULT > SENSITIVITY and so the drill/driver is de-energised.

Claims: A method of controlling a drill/driver, the drill/driver including a rotatable shaft arranged to mount a screwdriver and bit and a motor (38,39)for rotatably driving the shaft, the motor (38,39) being driven only during predetermined conduction periods, characterised in that the method comprises: (i) providing means that (a) monitors the conduction period required to maintain a constant motor speed within a given time period and (b) calculates first and second values, each of which is calculated using the said monitored conduction period, the first value being a function of the rate of change of the said conduction period necssary to maintain the given motor speed; and (ii) stopping the motor if the said first value has exceeded the said second value before the end of the given time period. A method according to claim 1, wherein the said second value is proportional to the difference between a predetermined reference level and the average value of the conduction period necessary to maintain a given motor speed. A method according to claim 1 or 2, wherein the motor is an a.c. motor and the motor is controlled by a triac (41), and the conduction period necessary to maintain a given motor speed and the rate of change of the said conduction period are monitored by measuring the delay before firing of the triac (41) during successive half cycles. A method according to claim 3, wherein the said time delay for successive half cycles is input and recorded as a count into successive stages of a serial stage memory shift register (114) in a microprocessor(30). A method according to claim 4, wherein the said first value, which is a function of the rate of change of the conduction period necessary to maintain a given motor speed, is provided by deducting the average of the eight most recently recorded counts from the average of the eight previously recorded counts. A method according to claim 3 or any claim dependent thereon, wherein the predetermined reference level used to calculate the second value is varied according to the screw size to be used, and/or the hardness of the workpiece into which the screw is being driven. A method according to any preceding claim, wherein the given motor speed is a steady motor speed.

Inventors: SHAW DAVID MARTIN, SHAW, DAVID MARTIN
Applicants: BLACK & DECKER INC, BLACK & DECKER INC.
Application year: 1994
Main classification: B25B23
IPC codes: B23P19, B25B23